The present invention relates to a frequency offset removal method and device to remove the frequency offset contained in modem reception signals, and more particularly to a frequency offset method and device that can remove frequency offset that are too large to be removed by an automatic carrier phase-controlling circuit disposed between an automatic equalizer and a decoder.
FIG. 1 shows an equalizing circuit in an automatic carrier phase control of the type used in conventional modem demodulation. The automatic carrier phase-controlling part (12) comprises a normalizing part (200) and a frequency offset computing part (202). The normalizing part (200) receives input in the form of equalizing signals from an automatic equalizer (24) and normalizing signals from a determining part (28), and outputs imaginary components in a remaining equalization signal which has been normalized. The frequency offset computing part (202) computes a frequency offset value .theta. which has occurred as a line fluctuation according to the remaining equalization signal, and outputs an offset correction signal that rotates inversely as much as the frequency offset value .theta.. The offset correction signal from the frequency offset computing part (202) is supplied to a multiplier (206) disposed in the automatic equalizer (24) to remove the frequency offset in an equalizing output signal (208). At the same time, it removes frequency offset in a remaining equalization signal from an adder (204), which has been supplied from the multiplier (208) and returned to a tap correcting part in the automatic equalizer (24).
However, removing the frequency offset using a conventional automatic carrier phase-controlling part has the problem that the frequency offset cannot be removed if the resulting error exceeds half of an angle in a symbol arrangement on the phase determination plane. For example, as shown in FIG. 2, if a frequency offset with .theta.=45.degree. or more to an adjoining symbol has occurred when four symbols (P1 through P4) are arranged on the same radius of the phase plane at a 90.degree. interval, then the set symbol of the reception symbol causing the frequency offset cannot be identified. As a result, the carrier phase may not correct the frequency offset, the frequency offset that has been caused from switching a network cannot be restored, and the modem freezes up. Demodulation using a narrow band with a low modulation rate such as in a subchannel, which makes each symbol cycle longer, is prone to large frequency offsets, which cannot be covered by an automatic carrier phase-controlling part in the manner of normal bandwidths.